The present invention relates to semiconductor packaging and, more particularly, to a semiconductor device having solder wettable corner leads or ‘flanks’ to facilitate inspection of solder joints when the semiconductor device is mounted on a substrate or circuit board using a surface-mount technology (SMT) process.
A typical semiconductor device comprises a semiconductor die attached to a lead frame flag. Bonding pads on the die are electrically connected to leads of the lead frame with bond wires. This assembly is encapsulated with a mold compound, which protects the die and wire bonds from environmental and physical damage. In some packages, the leads extend outward from the sides of the package, while in others such as the Quad Flat No leads (QFN) or Dual Flat No leads (DFN), the leads are flush with the sides of the package body. For example, during assembly, an array of packages are assembled simultaneously. After the molding or encapsulation step, individual devices are formed with a saw singulation step, where adjacent devices are separated using a saw. These device are non-wettable at their flanks due to the untreated copper surface that is exposed yet flush with the side walls of the device. That is, due to the manner in which the semiconductor packages are singulated with a saw blade, the surface of the exposed lead or flank is flush with the mold compound of the device such that solder does not readily climb-up or “wick” the flank of the package meaning that the QFN package is not flank wettable. This makes it difficult to inspect the solder joints after the package has been attached to a substrate or circuit board.
In order to overcome this deficiency, current leadless plastic packages with side solderable terminals have solderable flanks in the package side walls. However, this design cannot be applied to ultra-small packages (≦1 mm×1 mm) with multiple I/O terminals and a large I/O pitch (0.4 mm) because the package is not large enough to accommodate multiple terminals in the side walls.
Accordingly, it would be desirable to have an ultra-small leadless package with wettable flanks to facilitate inspection of solder joints when the package is mounted such as to a printed circuit board (PCB) using an SMT process.